1. Field of the Invention
The present invention relates to a technique for etching insulation films, more specifically an etching method suitably used in etching the inter-layer insulation films of a multi-layer interconnection structure.
2. Background Art
With the micronization of the interconnection layers and higher operational speed of semiconductor devices, the inter-layer insulation films are required to have low dielectric constants. Recently, organic low dielectric (low-k) films, especially SiOC-based low-k films are noted.
As the etching gases for the organic low-k films, mixed gases containing carbon fluoride-based gases as the main etching gases have been conventionally used. For example, in the plasma etching for forming via holes in an organic low-k film formed on a silicon nitride film, C4F8/Ar/N2 mixed gas, which is more selective to the silicon nitride film as the base film is dominantly used. However, C4F8/Ar/N2 mixed gas has a limitation in the etching rate of the low-k film and additionally has low selectivity to the resist film of the mask pattern. Accordingly, there is a risk that anisotropic (vertical) configurations may not be formed.
Then, in place of the above-described etching method using C4F8-based gas, the two-step method is proposed; i.e., first with CHF3/Ar/N2 mixed gas as the etching gas, the organic low-k film is etched down to a prescribed depth from the surface thereof in the openings of the mask, and then the etching gas is replaced by C4F8/Ar/N2 mixed gas to continue the etching, and when the etching arrives at the silicon nitride film, which is a lower layer or the base, the etching is completed.
In this two-step method, as the first processing, the plasma etching of the first step using CHF3/Ar/N2 mixed gas, because of a relatively small amount of the F radicals, the resist is not easily etched. On the other hand, the organic low-k film well reacts with radical species and ions in the plasmas to efficiently generate volatile products, such as SiFx, COx, CNH, etc. and is etched vertical configurations at high etching rates. However, because CHF3/Ar/N2 mixed gas has low selectivity to the base film (silicon nitride film), CHF3/Ar/N2 mixed gas must be replaced by C4F8/Ar/N2 mixed gas in the etching.